The United Nations' most senior human rights official said last night that the Israeli military may have committed war crimes in Gaza. The warning came as Israeli troops pressed on with the deadly offensive in defiance of a UN security council resolution calling for a ceasefire.

Navi Pillay, the UN high commissioner for human rights, has called for "credible, independent and transparent" investigations into possible violations of humanitarian law, and singled out an incident this week in Zeitoun, south-east of Gaza City, where up to 30 Palestinians in one house were killed by Israeli shelling.

Pillay, a former international criminal court judge from South Africa, told the BBC the incident "appears to have all the elements of war crimes".

The accusation came as Israel kept up its two-week-old air and ground offensive in Gaza and dismissed as "unworkable" the UN security council resolution which had called for "an immediate, durable and fully respected ceasefire".

Protests against the offensive were held across the world yesterday just as diplomacy to halt the conflict appeared to falter.

With the Palestinian casualty toll rising to around 800 dead, including 265 children, and more than 3,000 injured, fresh evidence emerged yesterday of the killings in Zeitoun. It was "one of the gravest incidents" since Israel's offensive began two weeks ago, the UN office for the co-ordination of humanitarian affairs said yesterday.

"There is an international obligation on the part of soldiers in their position to protect civilians, not to kill civilians indiscriminately in the first place, and when they do, to make sure that they help the wounded," Pillay told Reuters. "In this particular case these children were helpless and the soldiers were close by," she added.

An Israeli military spokeswoman, Avital Leibovich, said the incident was still being examined. "We don't warn people to go to other buildings, this is not something we do," she said. "We don't know this case, we don't know that we attacked it."

Despite the intense bombardment, militants in Gaza fired at least 30 rockets into southern Israel yesterday. Sami Abu Zuhri, a Hamas spokesman, told al-Jazeera TV: "This resolution doesn't mean that the war is over. We call on Palestinian fighters to mobilise and be ready to face the offensive, and we urge the Arab masses to carry on with their angry protests."

Israeli officials said they could not be expected to halt their military operation while the rockets continued and said they first wanted an end to the rocket fire and a "mechanism" to prevent Hamas rearming in future.

"The whole idea that Israel will unilaterally stop protecting our people when Hamas is sending rockets into our cities to kill our people is not a reasonable request of Israel," said Mark Regev, spokesman for prime minister Ehud Olmert. Israel wanted security for its people in southern Israel, he said, and dismissed suggestions his military might seek to topple Hamas, saying they were "not in the regime-change business".

Israeli public opinion still strongly favours the war. One poll of Jewish Israelis yesterday, by the War and Peace Index, said 90% of the population supported continuing the operation until Israel achieved all its goals.

Olmert held a meeting of his security cabinet, and on the agenda was discussion about whether to intensify the offensive by launching a fresh stage of attacks in which Israeli troops would invade the major urban areas of Gaza as more reservists were called up. There was no word on the outcome.

So far 13 Israelis have been killed in this conflict, of whom three were civilians.

Another 23 Palestinians were killed by the Israeli military yesterday. Seven from one family, including an infant, died when Israeli jets bombed a five-storey building in Beit Lahiya, in northern Gaza. There was heavy aerial bombing and artillery fire across the territory.

More than 20,000 Gazans have fled their homes in the north of the strip and thousands more in the south. In some cases Israeli troops have told them to leave, or dropped leaflets warning them to evacuate their homes. Some are even dividing their families between different addresses for fear of losing them all in a single air strike.

"Many people are leaving their homes and moving to the centre of the cities," said Abdel Karim Ashour, 53, who works with a local aid agency, the Palestinian Agricultural Relief Committee. He, his wife and their four children fled their house on the coastal road in northern Gaza on the third day of the conflict. He sent the four children to stay with his brother while he and his wife are staying at a friend's house. "We were in an area of heavy shelling, so we left and I divided the family to try to reduce the victims if we face any trouble. We try and keep in touch by telephone but there are problems with the network," he said. "We're just hoping for a ceasefire. If the fighting goes on there will be more victims."